1. Field of the Invention
This invention relates to the separation of oil-water mixtures. In one aspect, it relates to a method and apparatus for use in offshore operations whereby oil and water mixtures produced from an offshore well can be separated to permit the discharge of water into the sea.
2. Description of the Prior Art
Most oil and gas wells produce a certain amount of water or brine in conjunction with the oil and gas. A major problem faced in offshore oil operations is the economic separation of such oil, gas and water mixtures so that the water can be discharged back into the sea. The separated water returned to the sea must have a quality that complies with environmental discharge laws and regulations. For example, recent guidelines issued by the Federal Environmental Protection Agency requires that water discharged into the Gulf of Mexico may contain no more than 72 parts per million of oil. Conventional separation devices such as heater treaters normally discharge an aqueous effluent that typically contains 200 to 300 parts per million of oil and frequently as much as 1000 parts per million.
Numerous methods and apparatus have been employed by the offshore petroleum industry to reduce the oil concentration in produced water to environmentally acceptable limits. One approach has been to use settling vessels which rely on gravitational forces to separate oil-water mixtures. One such device specially adapted for offshore operations is the skim pile which is essentially long vertical pipe that is adjacent to or supported by the offshore production platform. The pile normally extends from the lower deck of the platform to near the seafloor. Waste water containing oil is continuously introduced near the lower end of the pile and is permitted to separate into a relatively thin layer of oil at the top and a clarified zone of water at the bottom. The oil is withdrawn from the top of the pile while clean water flows out of the open bottom.
The major limitation of skim piles is that they can only handle relatively low flow rates. If the flow rate of waste water is too high, oil droplets cannot rise against the motion of the fluid stream. For a given flow capacity of oil bearing waste water there will be a skim pile having certain minimum dimensions capable of separating the oil-water mixture. (See, for example, D. L. Thomas, World Oil, Aug. 1, 1968, pp. 66-69 which sets forth a method for sizing skim piles and other separators.) For all practical purposes, the diameter of the skim pile cannot exceed 60 inches. Beyond that size, special support structures would be necessary to support the weight of the pile and absorb its movement caused by wave and current action. For example, separate support structures have been designed for large diameter caisson vessels used to separate oil-water streams produced from offshore fields in Lake Maracaibo. Such special support frames are extremely expensive and can be the deciding factor in determining whether the entire offshore structure will be economical. For most situations, skim piles will rarely be used for flow rates greater than 100 gallons per minute.
For higher waste water flow rates other types of separation equipment have been employed, for example, coalescers and gas flotation units. Fibrous media coalescers utilize a filter element to absorb and coalesce oil particles from the mixture. (See U.S. Pat. No. 3,794,583.) These devices tend to get saturated with oil and plugged with suspended solids, necessitating frequent replacement or cleansing of the filter. Corrogated plate coalescers which have no moving parts and do not use a disposable coalescing unit also tend to get plugged by suspended solids. Dismantling the unit for clean-up is time consuming and requires the availability of a back-up unit to maintain continuous production from the oil and gas fields.
Gas flotation units of the type described in U.S. Pat. No. 3,884,803 utilize high speed impellers or pressurization systems to disperse gas into the oil-water mixture. The gas bubbles contact and adhere to the oil droplets thereby greatly increasing their buoyancy and the rate at which they will rise through the oil-water mixture. The major disadvantage of gas flotation units is that they usually require large moving parts such as mixers and complex auxiliary apparatus such as pressurization chambers.
There is, therefore, a need in the art for an offshore system which can efficiently and effectively separate oil-water mixtures at high flow rates to enable the discharge of a clean, environmentally acceptable effluent stream of water.